A traditional way of extracting gold from crushed ores has involved leaching the ores with cyanide, followed by recovery of the gold from the leachate solutions, often by absorption of the gold on activated carbon. The cyanide converts gold to a water-soluble coordination complex which can be leached from the matrix and separated from the residual solid ore. Copper, zinc and silver can be extracted in the same way, and are often present in the same ore bodies as gold. Unfortunately, cyanide is highly poisonous to most life forms and the cyanide process has consequently become very controversial and its use has been criticized or restricted in a number of countries, states and territories. The cyanide process also encounters a number of difficulties when used for the treatment of copper-bearing precious metal ores. The dissolution and oxidation of copper minerals that take place when such ores are subjected to the conventional leaching process consume larger quantities of cyanide and oxygen, reduce precious metal extraction yields and cause problematic precious metal recovery due to dissolved copper species.
An alternative to the cyanide process, which is more effective for ores with high copper values, involves the use of ammoniacal thiosulfate as a lixiviant for the precious metals. Thiosulfate forms a strong complex with gold (I) ions, i.e. [Au(S2O3)2]3−, and with ions of other precious metals. An advantage of this approach is that thiosulfate is essentially non-toxic, but unfortunately ammonia is also required to avoid passivation of the precious metals, to stabilize copper (II) and to increase the rate of precious metal dissolution. The use of high volumes of ammonia ensures a high leaching rate, but also makes the process less desirable for several reasons. For example, (a) ammonia is toxic to humans (when exposed to ammonia fumes), and especially to aquatic life when in solution; (b) ammonia is difficult to handle, transport and store; (c) the presence of ammonia increases the consumption or oxidation of the thiosulfate; (d) gold extraction rates may be reduced in the presence of some sulfide minerals; and (e) copper is still often not stable in ammonia-thiosulfate solution. For these reasons, alternatives to the use of ammonia in the thiosulfate leaching process have been investigated.
A number of methods have been attempted to overcome the problems of the ammoniacal thiosulfate leaching process. For example, one method involves the use of certain compounds, e.g. ethylenediaminetetraacetic acid (EDTA), to assist the leaching process. Another involves the use of deoxygenating conditions during thiosulfate leaching. However, all these methods still require the use of free ammonia to perform efficiently.
There are a few known thiosulfate processes that do not need ammonia, e.g. the process of U.S. Pat. No. 6,660,059 which issued to Ji et al. on Dec. 9, 2003. This process employs lixiviants that include at most only small amounts of copper and/or ammonia. To reach an acceptable leaching rate, this process needs to be conducted under conditions of high temperature and pressure.
In a paper presented at the XXIV International Mineral Processing Congress, in Beijing, China held on Sep. 24-28, 2008 (and published in the Proceedings of the meeting), W. T. Yen and C. Xia disclosed a leaching process employing a copper ligand such as EDA or DETA. These processes work with or without the presence of ammonia or ammonium.
However, the attempts made so far have not overcome all of the problems and further improvements are required. In particular, sodium thiosulfate leaching of highly sulphidic gold ores can be difficult due to the detrimental effects of some sulphides. Furthermore, sodium thiosulfate leaching processes appear to be slower than the cyanidation and the ammoniacal thiosulfate leaching processes.